As we wait for scientists to produce a COVID-19 vaccine, contract tracing is one of the most reliable ways to stop the spread. Now, Pennsylvania and Delaware have also released a new “exposure notification app” to let users know if they’ve been near someone who’s tested positive for the virus. New Jersey is piloting its app on college campuses. WHYY health and science reporter Alan Yu explains this could help reach people traditional contact tracing isn’t — but there’s a tradeoff: The privacy protections put in place to convince more people to download the app could make it much more difficult to know how effective it is.